Hidden Terminal Problem and
Exposed Terminal Problem in
Wireless MAC Protocols
Networking basics
Medium Access
Control (MAC)
Protocol
Characteristics of Wireless Networks

Multiplexing: in a mobile and wireless network,
the wireless medium is shared by many nodes.

Hence, multiple use of a shared medium is a
major challenge in wireless networking.

Most decisions for accessing the wireless
medium is made in the MAC layer.
Multiplexing

The wireless channels can be multiplexed in four
dimensions:
–
Time(t): A channel gets the whole frequency spectrum for
a certain amount of time.
–
Space(s): Same frequency can be reused when the base
stations are separated in space.
–
Frequency(f): The whole spectrum is separated into
smaller frequency bands.
–
Code(c):Each channel uses a unique code for transmitting.
Time Division Multiplex (TDM)




A channel gets the whole
frequency spectrum for a
certain amount of time.
Only one user for the
medium at a time.
Usually the throughput is
high even with many
users.
However, no two users
should use the medium at
the same time. Precise
synchronization
is
needed.
f
t
Space multiplexing : Cellular Networks

Same frequency can be
reused when the base
stations are separated in
space.

The reuse of frequencies
depend on signal
propagation range.

Example : fixed frequency
assignment for reuse with
distance 2.
Frequency Division Multiplex (FDM)




The whole spectrum is
separated into smaller
frequency bands.
A band is allocated to a
channel for the whole
time.
This is somewhat
inflexible if the traffic is
non-uniform.
An example is radio or TV
broadcast. The bandwidth
is wasted if a station is off
the air.
f
t
Code Division Multiplex (CDM)

Each channel uses a
unique code for
transmitting.

All channels use the
same frequency spectrum
at the same time.
c
t

However, signal
regeneration is very
complex and requires
complex HW/SW support.
f
Code Division Multiplexing

CDMA has ben adopted for the 3G mobile
phone technology.

CDMA is not very suitable for ad hoc networking
as we cannot expect specialized
hardware/software support at the nodes.

TDMA and its variations are most suitable for ad
hoc networking.
Demand Assignment Multiple Access (DAMA)

In a DAMA protocol, nodes first reserve slots
which they intend to use for broadcasting.

Each round of broadcast is preceded by a
reservation round.

DAMA protocols are widely used in satellite
communication and increasingly being used in
wireless networking.
An example of Time Division Multiplexing

CSMA/CD: Carrier Sense Multiple Access with
Collision Detection

When a node wants to broadcast, it checks
whether any other node is broadcasting (senses
the carrier).

A node broadcasts when no other node is
broadcasting. Otherwise, it tries later at a
random interval.
CSMA Problems in Wireless Medium

Collision detection is easy in wired networks but
difficult in wireless medium.
Transmission
Range of S
R
S

Collision avoidance to reduce wasted transmissions
CSMA Problems in Wireless Medium

With only one antenna/radio, nodes can only
listen or send.

Full duplex radios are extremely expensive.

CSMA gives rise to hidden terminal and exposed
terminal problems.
Message Loss due to Collision

Using CSMA in wireless medium results in
message loss and requires retransmission of
lost messages.

A node spends much more energy while
receiving or transmitting messages. Hence,
retransmission wastes a lot of energy.
Hidden Terminal Problem
Other senders’ information are hidden from
the current sender, so that transmissions at
the same receiver cause collisions.

R
S1
S2
MACA – Multiple Access Collision Avoidance

Use of additional signaling packets
–
–
–
Sender asks receiver whether it is able to receive a
transmission - Request to Send (RTS)
Receiver agrees, sends out a Clear to Send (CTS)
Sender sends, receiver Acknowledgements (ACKs)
RTS
S1
R
1
3
2
CTS
R
S1
S2
DATA
4
ACK
S2
Detect
Collision
time
Find Transmission
Complete
Exposed Terminal Problem

The sender mistakenly
think the medium is in
use,
so
that
it
unnecessarily defers
the transmission.
R1
S1 1
S2
R2
S1
R1
S2
R2
CTS
RTS
2
3
DATA
Find medium in use
Wait until medium is clear
time
MACA – continued

Synchronization
–
–
There is a global clock. Every node knows the current time.
There is a global schedule. Every node knows the schedule.
Name
Time
Tom
1:00 pm – 2:00 pm
Peter
2:00 pm – 3:00 pm
John
3:00 pm – 4:00 pm
MACA – continued

When a node hears an RTS from a
neighboring node, but not the
corresponding CTS, that node can
deduce that it is an exposed terminal
and is permitted to transmit to other
neighboring nodes.
2
R1
S1 1
S1
R1
S2
R2
CTS
DATA
RTS
Exposed
Terminal S2
5
RTS
4
3
R2
t1
t2
t3
CTS
t4
6
t5
DATA
t6
time
MACA – continued

Collision handling
–
If a packet is lost (collision), the node back
off for a random time interval before retrying
Quiz
N2
N1



N3
If N1  N2, can N3  N2 simultaneously?
Why this situation happens in wireless network?
How to solve it?
RTS
S1
R
1
3
2
CTS
N1
DATA
4
N2
ACK
N3
S2
time
Quiz
N1



N2 N3
N4
If N2  N1, can N3  N4 simultaneously?
Why RTS/CTS mechanism do not allow N3  N4?
How to solve it?
2
R1
S1 1
CTS
DATA
RTS
N2
S2
RTS
N1
3
R2
t1
t2
t3
5
N3
N4
4
CTS
t4
6
t5
DATA
t6
time
Quiz
N2
N3

Is there any collision happens?

What kind of problem (hidden/exposed) shown in the figure?
Node 1
N1
DATA
RTS
CTS
Node 2
CTS
Node 3
Node 4
N4
RTS
RTS
time
Quiz

If N1  N2, can N3  N4?
N2
N1

N3
N4
How to solve the problem?
Node 1
RTS
DATA
CTS
Node 2
Node 3
Node 4
RTS
DATA
CTS
time
Summary

Hidden Terminal Problem
–

Other senders’ information are hidden from
the current sender, so that transmissions at
the same receiver cause collisions.
Exposed Terminal Problem
–
The sender mistakenly think the medium is in
use, so that it unnecessarily defers the
transmission.
Conclusions

A Perfect MAC Protocol
–
–

Collision avoidance to reduce wasted
transmissions
Cope with hidden terminal problems
Allow exposed terminals to talk
Reasonable fairness
No MAC protocol does all this!